Enzyme-based biosensors are devices in which a signal from an analyte-concentration-dependent biochemical reaction is converted into a measurable optical or electrical signal. Amperometric, enzyme-based biosensors typically employ two or three electrodes, including at least one measuring or working electrode and one reference electrode. The working electrode is composed of a non-corroding carbon or a metal conductor and is connected to the reference electrode via a circuit, such as a potentiostat. The working electrode typically includes a sensing layer in direct contact with the conductive material of the electrode. The sensing layer may include an enzyme, an enzyme stabilizer such as bovine serum albumin (BSA), and a crosslinker that crosslinks the sensing layer components. Alternatively, the sensing layer may include an enzyme, a polymeric mediator, and a crosslinker that crosslinks the sensing layer components, as in a “wired-enzyme” biosensor.
In an example of an amperometric, enzyme-based, glucose biosensor, the sensor utilizes glucose oxidase, which catalyzes the oxidation of glucose by oxygen in a sample of body fluid and generates gluconolactone and hydrogen peroxide, whereupon the hydrogen peroxide is electrooxidized and correlated to the concentration of glucose in the sample (Thom-Duret et al., Anal. Chem. 68, 3822 (1996); and U.S. Pat. No. 5,882,494 of Van Antwerp et al., filed on Aug. 28, 1995). In another example of an amperometric, enzyme-based, glucose biosensor, a polymeric redox mediator “wires” the reaction center of glucose oxidase to an electrode and catalyzes the electrooxidation of glucose to gluconolactone. The principle and the operational details of such a “wired-enzyme” biosensor have been described (Csoregi, et al., Anal. Chem. 1994, 66, 3131; Csoregi, et al., Anal. Chem. 1995, 67, 1240; Schmidtke, et al., Anal. Chem. 1996, 68, 2845; Schmidtke, et al., Anal. Chem. 1998, 70, 2149; and Schmidtke, et al., Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 294).
The operation and performance of an amperometric biosensor, such as those just described, may be complicated at high rates of analyte flux. For example, at high rates of glucose flux, an amperometric glucose biosensor may be kinetically overwhelmed, such that the relationship between the concentration of glucose in a sample fluid and the response from the biosensor becomes non-linear. This kinetic problem may be solved by the interposition of an analyte-flux-limiting membrane between the sample fluid and the sensing layer of the biosensor, as described in the above-mentioned U.S. Patent Application Publication No. U.S. 2003/0042137 A1 of Mao et al. Still, the development of analyte-flux-limiting membranes, such as glucose-flux-limiting membranes, has not been without its challenges. Many known membranes have proved difficult to manufacture and/or have exhibited properties that limit their practical use, such as practical use in a living body.
Various biosensors have been designed to operate partially or wholly in a living body. Indeed, clinical use of such biosensors has been a significant step toward helping diabetic patients achieve tight control over their blood glucose levels. However, some of these biosensors have been known to provide spurious, low-glucose-reading incidents, particularly during periods of stillness, such as when a subject is asleep. For example, Metzger et al. and McGowan et al. have demonstrated that the CGMS continuous glucose monitoring system of Medtronic MiniMed (Northridge, Calif.) provides such spurious, low-glucose-reading incidents. (See Metzger et al., Reproducibility of glucose measurements using the glucose sensor, Diabetes Care, July 2002, Vol. 25, 1185-1191; and McGowan et al., Spurious reporting of nocturnal hypoglycemia by CGMS in patients with tightly controlled type 1 diabetes, Diabetes Care, September 2002, Vol. 25, 1499-1503.) These low-glucose-reading incidents are very problematic, particularly in the monitoring and treatment of a diabetic subject, as they indicate that a subject is hypoglycemic when the subject is not. As an example, when a spurious, low glucose reading is used as a signal to control insulin dosage, a subject may receive an improper or a reduced dose of insulin and thus be put at risk or in actual danger.
The cause of low-glucose-reading incidents has not been understood and no specific hypotheses as to the cause of these incidents have been put forward for consideration, testing, analysis, or evaluation. For example, while Metzger et al. and McGowan et al. noted that the above-mentioned CGMS system of Medtronic MiniMed measures interstitial fluid glucose levels, rather than capillary glucose levels, and that there may be a slight time lag between the two if the blood glucose level is changing rapidly, this does not adequately explain the occurrence of low-glucose-reading incidents. (Id.) Further, neither Metzger et al. nor McGowan et al. provided a hypothesis as to the cause of these low-glucose-reading incidents.
Further development of biosensor components and biosensors is desirable.